1. Field of the Invention
The present invention relates generally to mail processing systems and equipment and, more specifically to a transport/stacker module for a flat mailpiece sorting system.
2. Description of the Related Art
A typical automated mail processing system includes a mail induction system and a transport system, the mail induction system feeds mailpieces to the transport system which includes an endless loop conveyor consisting of a succession of mailpiece carriers. The induction system delivers a mailpiece to each of the carriers as they are conveyed along a conveyor path of oval or "racetrack" configuration. A plurality of sorting sites are distributed along the conveyor path. A sorting bin, positioned at each sorting site, is designated for a particular zip code, mailstop, address, etc. As the carriers move in succession past the sorting sites, separate rake mechanisms, respectively stationed thereat, are selectively actuated to rake mailpieces from the carriers into the appropriate bins positioned beneath the conveyor path. Mail transport systems of this conventional design require considerable floor space, are not readily expandable to increase the number of sorting sites, and are limited in sorting rate or throughput.
In the above-cited related application, a mail transport/stacker module, as disclosed therein, includes an endless loop conveyor for conveying a continuous succession of mail carriers in a serpentine conveyor path including a plurality of vertically spaced, horizontal conveyor runs. Sorting sites are distributed along the horizontal conveyor runs. Since the carriers move in alternating directions as they travel along successive horizontal conveyor runs, the carriers are equipped with doors at each end. As the conveyors move along a horizontal conveyor run in one direction, the trailing one of the doors of each carrier is in an open position, while the leading door of each carrier is in a closed position. A sorting rake at each sorting site is selectively actuated to rake a mailpiece from the carriers through their trailing, open ends into an underlying bin. The closed leading door acts as a wind screen to prevent relative wind from dislodging mailpieces from the carriers and also acts as a deflector to reliably direct a mailpiece, raked from an immediately preceding carrier, downwardly into an underlying bin. As each carrier transitions from one horizontal conveyor run to the next, its trailing door is closed. Then, as each carrier begins to move through this next horizontal conveyor run, the other, now trailing carrier door is opened to accommodate the raking of mailpieces from the carriers into the bins at the sorting sites distributed along this next horizontal conveyor run.
Sorting rate or throughput of mailpieces is directly related to conveyor speed and carrier pitch. Carrier pitch is determined by the length of the carriers, i.e., the carrier longitudinal dimension in the direction of conveyor motion, plus the length of the longitudinal gap between consecutive carriers. Carrier length must be somewhat larger than the maximum mailpiece size to be transported so that the mailpieces can lie flat on the carrier floor. The gap dimension between carrier must be sufficiently large, such that the longest and stiffest mailpieces that are to be sorted will be reliably directed downwardly into a sorting bin by the closed leading door of the immediately trailing carrier.
Maximum conveyor speed is determined by the forces on the mailpieces associated with motion, namely aerodynamic forces and inertial forces. Excessive conveyor speed leads to mailpieces being "blown off" the carriers and/or ejected from the carriers by centrifugal forces as they execute the 180.degree. turnarounds to transition from one horizontal conveyor run to the next horizontal conveyor run of the serpentine conveyor path. The aerodynamic limiting speed is dependent on the characteristics of the mailpiece, e.g., large size, light weight mail pieces are blown off at a lower speed than small size, heavier pieces. The inertial limiting speed, on the other hand, is the speed at which mail pieces are ejected from the carriers in the 180.degree. turns when the centrifugal force acting on the mailpieces exceeds their weight.